first gear frustration

hi, new to the site. i have a 95 lasabre with 40,000 pampered miles. when i received the car everything worked fine. about 1,500 miles later first gear worked about 50% of the time when in Drive. even shifting to 1 on the column only engaged 2nd gear (50% of the time) ive checked the fluid level, flushed and changed the filter and fluid, checked all wiring and ensured that linkage was properly shifting thru all gears. any help or suggestions would be great. thank you

Could be that your transmission is slipping out take it to a transmission shop as soon as you can.